In recent years, the deterioration in the external appearance of paint films due to chemical and physical causes, such as etching by acid rain and scratching during cleaning of the paint film for example, has been seen as a problem in the field of automobile paints, for example.
Acrylic-melamine based top-coat paint compositions in which .epsilon.-caprolactone modified acrylic resin and specified low molecular weight polyols are used conjointly in the essential structural material of the paint film and in which soft segments are introduced by hardening with an alkoxy monomeric melamine (for example, Japanese Patent Kokai 64-66274, (1989)) have been suggested as a means of resolving these problems.
However, the abovementioned paint compositions are such that the paint film forming resin itself is modified with .epsilon.-caprolactone and so the paint film is softened and the scratch resistance can be improved, but the acid resistance is inadequate because alkoxy monomeric melamine is used, and this is inappropriate for dealing with severe acid rain in the future.
Furthermore, top-coat paint compositions which have .epsilon.-caprolactone modified acrylic resin and non-yellowing type blocked polyisocyanate compounds as essential components (for example, Japanese Patent Kokai H2-305873 (1990)) have also been suggested with the same end in view.
However, although both scratching resistance and acid resistance are improved with the abovementioned compositions, a blocked isocyanate compound is used for the crosslinking agent and so the paint film in fact liable to yellowing even though it is said to be a non-yellowing type, and in practice the non-yellowing properties are inadequate.
On the other hand, hardenable compositions which contain poly-functional hydroxyl group containing materials, 1,3,5-triazine-2,4,6-tris-carbamic acid esters, and acid hardening catalysts have been disclosed in Japanese Patent Kokai H6-228305 (1994), and it is stated that paint films which release little formaldehyde and which have excellent acid resistance are obtained.
However, these compositions are inadequate in terms of weather resistance and thermal stability, and paint film defects such as cracking and loss of gloss due to exposure to ultraviolet radiation are liable to occur with paint films which have been formed under hardening conditions of temperature above 140.degree. C. in particular.
The aim of this invention is to provide heat-hardenable paint compositions with which paint films which have excellent scratching resistance, acid resistance, yellowing resistance and weather resistance can be formed in order to resolve the problems outlined above.